Known ropes made of textile fibers consist of twisted or laid strands or of hollow braided ropes or of ropes comprising a core and a sheath (called Kern-Mantel ropes). In ropes of the last-mentioned kind, the core consists of parallel or braided yarns or ply yarns and the mantle consists of a braided tube, which encloses the core.
All known ropes of the kind described hereinbefore can more or less easily be ruptured when they are sharply bent or moved around sharp edges or when they are subjected in operation to high alternating bending stresses, particularly with small radii.
Laid-open German No. 22 22 312 discloses a highstrength rope, which comprises load-carrying threads of synthetic resin, and reinforcing threads which are highly stretchable but have not been stretched or have not been stretched to the highest permissible extent. The reinforcing threads are provided in such quantity and arrangement that in case of an excessive loading of the rope, i.e., when the permissible tensile stress is highly exceeded, the potential energy of the stress is dissipated by the synthetic threads in steps. When the elastic rope is used in the operation of ships, the reinforcing elements are intended to avoid accidents when the rope is ruptured because the ends of the rope would otherwise whip around.
Laid-open German Application No. 24 55 273 discloses a crane rope which consists of a plurality of laid or braided strands and includes elements, such as strands, ply yarns or filaments, which have the same length or almost the same length and are made of a synthetic resin, particularly polyamide, which has a small elongation at break. That rope comprises also internally disposed, shorter elements, such as core strands, core ply yarns or core filaments, which consist of a synthetic resin having a higher elongation at break, such as stretched polyamide, polyester or polypropylene. The rope or each strand thereof may optionally be surrounded by a single sheath layer consisting of flexible synthetic resin, such as polyurethane. Rope elements consisting of different materials are used in such ropes so that the high-strength rope elements having the same length will be subjected to approximately equal stresses when the rope is loaded and the shorter rope elements having a higher elongation will also contribute to the tensile strength of the rope and will not rupture under relatively small loads. The life of the rope is increased by the provision of an external sheath or by an impregnation of the rope with a wear-resisting synthetic resin. In practice, a single sheath layer has the disadvantage that it will not properly adhere to the remainder of the rope and will crack or flake off after a short time of use. Similar disadvantages are encountered in the use of plastic-covered wire ropes, such as are disclosed in German Patent Publication No. 12 21 926.
Laid-open German Application No. 15 10 114 discloses an elastic rope which is intended for safety purposes and comprises a sheath of braided wires and a core which consists of synthetic resin, particularly polyurethane, which has been foamed in the sheath after the latter has been made. In the manufacture of that rope the sheath is preferably coated with a varnish or paint in order to ensure the required resistance to corrosion. Owing to the low strength of its core, such rope has only a low tensile strength and under alternating bending stresses may become unusable by a rupture of the core. For this reason that rope is mainly intended for special purposes, for instance, as a flexible crash guard used instead of a guard rail at the edges of roads.
Laid-open French Application No. 21 66 695 discloses a climbing rope which comprises a core and two sheaths, which surround the core and consist of the same materials and have the same structure. Two sheaths are provided in order to increase the safety and the wear resistance because it is assumed that the other sheath will wear first before the wear of the inner sheath begins. But that design does not result in a higher tensile strength of the rope when the latter is subjected to high tensile stresses while it is bent around a sharp edge.
The ropes which are disclosed in the prior art discussed hereinbefore have been developed for purposes other than those for which the ropes in accordance with the invention is intended.